Microelectromechanical (MEM) technology generally involves the fabrication of small mechanical devices on a substrate (usually silicon). The various microstructures of a MEM system may be formed using a variety of appropriate processes. An example of such a process is a surface micromachining process that generally entails depositing alternate layers of structural material and sacrificial material on an appropriate substrate, which generally functions as a foundation for the resulting microstructure(s). Various patterning operations may be executed on one or more of these layers (usually, but not always, before the next layer is deposited) so as to define the desired microstructure(s). Generally, after at least a portion of the microstructure(s) has been defined in an appropriate manner, such as by the process described above, the various sacrificial layers (if such layers are present) may be removed by exposing the microstructure(s) and/or the various sacrificial layers to at least one etchant to “release” the resulting microstructure(s).
MEM-based systems can generally include suspension assembly microstructures that support movable elements, such as electrostatic elements of electrostatic actuators. These electrostatic actuators may have, for example, both stationary and moveable electrostatic combs, which, in combination, may function to provide power/motive sources to microstructure(s) of microelectromechanical systems. In the case of a suspension assembly being utilized in an electrostatic comb actuator application, one or more moveable combs of the electrostatic comb actuator(s) generally may be attached to the suspension assembly to enable each moveable comb to move toward and/or away from its accompanying stationary comb.
However, various problems have been associated with conventional suspension assemblies. Take, for example, the case of a moveable element such as a moveable electrostatic comb associated with a conventional suspension assembly. Generally, each electrostatic comb has a base beam with a plurality of comb fingers extending therefrom. An increasing amount of voltage is typically needed to urge a moveable comb (via movement of the suspension assembly) toward a corresponding stationary comb to produce a resultant actuation (e.g., movement of a mirror in an optical switch application). More specifically, the application of voltage to, for example, the stationary comb of the actuator creates a variety of attractive forces between the moveable and stationary combs. A first of such attractive forces is a “comb force” generally defined by the change in capacitance per unit of displacement that arises between each side of each movable comb finger and sides of the stationary fingers by which it passes. Typically this attractive comb force will generally vary with respect to the square of the applied voltage. This comb force also is generally the main force that affects the positioning of the moveable comb with respect to each corresponding stationary comb in electrostatic actuator assemblies.
A second of such attractive forces is a “parallel plate force” generally defined by the attraction of each finger of the moveable comb toward adjacent stationary comb fingers disposed on each side of the moveable comb fingers. Since this parallel plate force is generally oriented in direction that is substantially perpendicular to the movement of the moveable comb, and since the stationary comb elements are typically equidistantly spaced from each corresponding moveable comb finger, the parallel plate force is generally offset and can (for the most part) be ignored.
A third of such attractive forces is a variation of the parallel plate force referred to as a “parasitic tip force”. This parasitic tip force generally refers to the attraction of each free end of each moveable comb finger toward the base beam of the stationary comb (and/or the attraction of each free end of each stationary comb finger for the base beam of the moveable comb). Accordingly, the parasitic tip force is generally oriented in direction that is substantially parallel to the movement of the moveable comb. This parasitic tip force generally is insignificant to the function of the electrostatic comb actuator until the free ends of the moveable comb fingers reach (or surpass) some minimum threshold distance of separation with respect to the base beam of the corresponding stationary comb.
Even though the attractive forces of an electrostatic actuator are generally opposed by an attached suspension assembly (typically providing some sort of restorative force), the parasitic tip force, in combination with the comb force, tends to overcome restorative forces of conventional suspension assemblies. In other words, as the free ends of the moveable comb fingers reach or surpass this minimum threshold distance of separation from the base beam of the stationary comb, this parasitic tip force causes an increase in the attractive forces of the stationary comb. It can be said then that the total attractive force of the stationary comb increases in a substantially non-linear fashion at least when the free ends of the moveable comb fingers reach or surpass the minimum threshold distance.
As a result, conventional suspension assemblies have allowed their attached moveable combs to remain locked into an interdigitated engagement with the corresponding stationary comb, thus rendering the electrostatic actuator at least temporarily inoperable. While conventional electrostatic actuators have traditionally relied on a reduction in the voltage applied to the stationary comb and/or the restorative spring force of the accompanying suspension assembly to provide a restoring force to draw the moveable comb(s) back out of (or at least to a lesser degree) interdigitation with the stationary comb(s), the designs of conventional suspension assemblies have not been able to successfully address the occurrence of runaway conditions and associated adhesion/stiction between the fingers of the stationary and moveable combs (or other opposing elements of a MEM system). In other words, once a moveable comb has been urged toward a corresponding stationary comb (generally via voltage applied to the stationary comb), the moveable comb may “snap” into and maintain, at least momentarily, an interdigitated actuation relationship with the stationary comb even after the applied voltage has been reduced. Stated another way, the designs/configurations of conventional suspension assemblies have not been successful at combating the additional parasitic force to avoid runaway conditions or, where such conditions have occurred, to overcome the tendency for corresponding moveable and stationary combs to stay “stuck” together until the applied voltage has been reduced below some threshold value. Several attempts have been made to combat this problem. For example, some attempts have included varying the length and/or thickness of spring arms associated with the suspension assembly, however such attempts have generally proven unsuccessful.
An additional consideration is that the “space” (or “real estate”) on a base substrate to which a MEM system is formed is generally limited. Accordingly, MEM systems are continually being designed to reduce the space occupied by electrostatic actuators. A significant amount of the designs include moving the combs elements of the actuator assemblies closer together, requiring even greater control of the comb elements. Accordingly, it would be desirable to provide a suspension assembly that is capable of addressing both the comb forces and parasitic forces associated with electrostatic comb actuators.